clrscr for Atari 5200 default conio screen

diff --git a/libsrc/atari5200/clrscr.s b/libsrc/atari5200/clrscr.s
new file mode 100644 (file)
index 0000000..041f34a
--- /dev/null
+++ b/libsrc/atari5200/clrscr.s
@@ -0,0 +1,34 @@
+;
+; Christian Groessler, May-2014
+;
+; void clrscr (void);
+;
+
+        .export         _clrscr
+        .include        "atari5200.inc"
+        .importzp       ptr1
+
+SCRSIZE =       480             ; 20x24: size of default conio atari5200 screen
+
+_clrscr:lda     SAVMSC          ; screen memory
+        sta     ptr1
+        lda     SAVMSC+1
+        clc
+        adc     #>(SCRSIZE-1)
+        sta     ptr1+1
+        lda     #0              ; screen code of space char
+        ldy     #<(SCRSIZE-1)
+        ldx     #>(SCRSIZE-1)
+_clr1:  sta     (ptr1),y
+        dey
+        bne     _clr1
+        sta     (ptr1),y
+        dex
+        bmi     done
+        dec     ptr1+1
+        dey
+        jmp     _clr1
+
+done:   sta     COLCRS_5200
+        sta     ROWCRS_5200
+        rts